Instrument Landing Systems (ILS) became available in the 1940s. The first of these in the UK was the ‘Standard Beam Approach (SBA) which operated at 45MHz, providing azimuth guidance only. This SBA service was terminated in the mid 1950s when a BBC Television service was launched on the same frequency band.
The ILS System
Since then ILS, with both azimuth and elevation guidance has become the ‘norm’. The theory of operation for all systems is similar, i.e. three basic elements are required, a ‘Localiser’ for azimuth angular guidance, a ‘Glidepath’ for elevation angular guidance and Marker Beacons, giving over-flying aircraft coded signals at set known distances from touch down. The main system elements produce an RF carrier frequency (vhf for the Localiser and uhf for the Glidepath), amplitude modulated, with very stable 90 Hz and 150 Hz frequencies fed to an aerial system, so constructed and configured that its elements produce a set of composite radiation patterns. Specially designed aircraft receivers give indicated angular deviation from the required path to the pilot and by measuring the DDM (difference in depth of modulation) between the two modulation frequencies, thence a zero DDM reading in the aircraft indicates that the aircraft is positioned on the airfield centre line or the extended centre line in the air. Any deviation from zero DDM requires the plane to be flown either left or right to obtain zero DDM and line up with the runway centre line, this equipment is known as the Localiser (vhf). The Glidepath (uhf) equipment radiates an RF signal modulated in a similar way to the Localiser, but through its aerial configuration produces a radiated pattern in a vertical plane, thus giving the approaching aircraft guidance information regarding its elevation above ground. The elevation information guidance is usually set to give the aircraft an angle of descent of 3° from touchdown.
Plessey Radar and ‘Navaids’
The Decca Navigator Company, from which Decca Radar grew, was formed specifically to produce what became’ The Navigator’ system for long range positioning of aircraft and ships at sea. But the company did not then venture into airfield navigational aids. It was not until the end of 1969, after Standard Telephone and Cables Ltd (STC) decided to close their navigation aids business (part of its Radio/Navaids Division at New Southgate, North London) that Plessey Radar’s involvement with Radio Navaids started. The closure of the STC Navaids business was a financially based decision taken by the parent Company, ITT of the USA. Many STC products were offered up for sale. Plessey Radar purchased the design, development and manufacturing rights for various Radio/Navaids systems. Of these, the main acquisition was the Instrument Landing System (ILS) comprising the STAN 37 Localiser, STAN 38 Glidepath and STAN 39 Marker Beacon. Production facilities for these products were quickly established by Plessey Radar, some at Cowes and the balance at Addlestone, this to provide Support and Sales Services for Instrument Landing Systems which were then in service or, in several cases awaiting installation and commissioning, all round the world. In time a new Company was created, based at the Addlestone site and was to be known as ‘Plessey Navaids’.
STC had entered the ground ILS market with the launch of its STAN 7/8/9 equipment, in 1959, and was installed widely at airports around the world. The system was universally respected for providing stable, reliable and accurate aircraft landing guidance. The products used vacuum tubes to provide the necessary radio frequency power for the transmitters and bulky cable distribution networks to feed the wide aperture antennas required to give the desired quality of guidance. By the early 1960s transistors were becoming available that could replace the vacuum tubes and printed circuit ‘ stripline’ offered better stability and lower loss than cables for the distribution networks. It was at this time that the Civil Aviation Authority issued its requirement for an ILS capable of providing guidance integrity to support fully automatic commercial blind landing operations. To meet this requirement STC designed the STAN 37/38/39 ILS. The first deliveries of the new system were made available in 1968.The ILS provides radio guidance information (azimuth and elevation) to suitably equipped aircraft from a distance of 25nautical miles to their final approach. In February 1971, a STAN 37, 38 and 39 ILS System was installed on Runway 28L at Heathrow Airport. After long and extensive trials by the CAA it became the first ILS system to be certified as meeting the International requirements (ICAO: Annex 10, Amendment 48) for a Category 3 Landing System. The product could be used as a blind landing facility, allowing appropriately equipped aircraft fitted with specialized receivers and associated special to type electronic and mechanical equipment, to make automatic approaches down to and along the surface of the runway.
Freedom from deviations in the guidance signals is crucial to ILS installations. Such deviations can be caused by reflections from buildings, such as aircraft hangars, overhead power cables and undulating terrain on the approach path outside the airfield. A number of antenna configurations were available for Localiser and Glidepath antenna to mitigate the effects of such site conditions. The STAN 37, 38 and 39 was part of a family of various designs of aircraft landing systems, going back to the 1940s, and was an upgraded design using state-of-the-art transistor technology, replacing the earlier STAN 7, 8 and 9. The heart of this system and its predecessors is a very reliable and stable modulator, (a constant impedance mechanical modulation system), producing the 90 Hz and 150 Hz modulating frequencies. These modulations were produced by a mechanically balanced modulator providing an input-impedance that was constant, which in turn eliminated course shift due to phase change between modulating frequencies and so virtually eliminated inter-modulation. The STAN 39 Marker Beacon system consisted of 3 identical installations of low power transmitters (75.0Mhz), each of them modulated by one of 3 different tones and Morse code, mounted at fixed universal distances (as inner, middle and outer marker beacons) from touchdown. The radiation upwards, in a circular cone on the extended runway centre line sourced coded audible tones, to be heard in the aircraft cabin, first alerting the pilot that he was 3.9nm (Outer Marker/400Hz dashes) from touchdown. Likewise the other two marker beacons gave similar urgent warnings to the pilot. The Middle Marker positioned at 3.5nm transmiting a 1300Hz series of dots and dashes, the Inner Marker at 1000m transmitting a 3000Hz tone. In keeping with its state-of-the-art design philosophy, the STC design for the STAN 39 marker beacon made extensive use of ceramic dual in-line package (“DIP”) integrated circuits. The fast pace of integrated circuit development at that time meant that these components were soon obsolescent and a decision was taken to buy marker beacons from the Norwegian NERA Company.
With the benefit of years of experience and specialisation gained from the design and development of STAN Systems, the Plessey PLAN 17,18 ILS systems were brought to the market, each element of an up to date ‘solid-state’ and modular design, whilst still performing to the original operational specification. Much enhanced were the reliability, stability, accuracy and beam monitoring over the earlier equipments. Also seen as plus points for the new products were their greater cost effectiveness, being easier to install and to commission, test and maintain. Besides the many advantages gained by using solid state modular technology, another major improvement, available in the PLAN 17,18 systems was the introduction of a Phase Locked Loop electronic modulator, producing very stable modulation frequencies (90Hz and 150Hz).
With the introduction of the modular construction came the ability to build up, over time, an increased complexity and capability. To provide the pilot with additional landing information i.e. distance and range from touchdown, an optional DME (Distance Measuring Equipment) could be co-located in the Glidepath Equipment Room. DME is a system usually co-located with ILS or DVOR providing accurate distance in the air from a base location, as in the case of the PLAN 18 (Glidepath) distance to touchdown. If required, a (Butler) DME would also be supplied.
The Plessey PLAN 17, 18 and 19 is currently in operation at many airports around the world.
MLS featured in the history of the Cowes site for more than two decades, beginning in the 1970’s with the Doppler MLS programme. When the ICAO adopted a competing ‘ scanning beam’ technology, Plessey Radar went on to produce what was to be known as P-SCAN.
The Doppler Microwave Landing System (DMLS) never became a Plessey product but ranks along side many of the more familiar radar projects conducted on the Cowes Site.
DMLS was a collaborative research and development programme led by RAE at Farnborough. The CAA and Plessey Radar were joint participants. The requirement specification was calling for an improved all weather aircraft landing aid. Plessey Radar was the prime contractor. 1971 would see ICAO issue an operational requirement to meet a projected increase in air traffic density and runway utilization throughout the world.
Following theoretical evaluation all parties agreed that any new system would have to move from existing ILS operating frequencies, in VHF and UHF bands, to a much higher (microwave) frequency.
ICAO established an international all weather operations panel (AWOP), this to adjudicate the merits of the various systems that might be proposed.
UK technologists believed that encoded airspace as a function of frequency would be more immune to distortion. To this end Doppler MLS (DMLS) was conceived around ground based antennas designed to radiate a signal originating from a point source whose phase centre is caused to move along a straight line. As a result the signal has a Doppler shift uniquely dependent on the angle from which it is received. The required ‘moving source’ is achieved by energising each element of a linear array antenna in rapid sequence. A British engineer working at the STC Company in the UK first proposed this Doppler scan principle in the mid 1960’s.
Plessey Radar was responsible for designing and building a DMLS proof-of-principle demonstrator. The ground sub-systems were built at Cowes with the airborne equipment coming from Southleigh, West Leigh and Roke Manor.
The demonstrator ground equipment had the same two functions as ILS, i.e. Azimuth (Localiser) and Elevation (glide-slope). Transmitter signal quality was monitored using sensors integral to the antennas also at locations remote from the equipment. Both antennas had apertures of 90 wavelengths (approximately 5 meters.)
After its building and test, on the Cowes site at Somerton, DMLS was installed on runway 09-27 at RAE Bedford.
Flight trials were conducted by RAE using an HS748 (Andover) fitted with the DMLS receiver equipment. The DMLS system was designed to achieve an angular accuracy of 0.01degree. Results from the Bedford trials proved that the principles on which DMLS was based were sound and that the required accuracy of 0.01degree could be achieved. To put this in perspective 0.01 degree is equivalent to a displacement of 11inches (280mm) at a range of l mile.
Fielding DMLS at more demanding locations, regarded as unsuitable for ILS, was the next phase of the programme. To enable trials at the airfields chosen by RAE, the CAA and later by ICAO, a new 54 wavelength Elevation Antenna was built. This was lighter and more compact. Trials at Bedford were then completed and the DMLS system was installed and successfully demonstrated at the following international airports: Gatwick UK, Manchester UK, Stanstead UK, Bern Switzerland, Tehran Iran, Brussels Belgium, Kristiansand Norway, Dorval Montreal and Dulles Washington.
The choice between the two candidate systems (DMLS and TRSB) was made at a full meeting of ICAO (an organ of the UN) in Montreal in April 1978. Their decision to adopt TRSB, so strongly advocated by the USA, brought the UK MLS programme to an end. BUT the entire adjudication process was challenged by all parties that comprised the UK team. Senator Barry Goldwater instigated a Congressional hearing on the matter to the effect that it was believed that the Lincoln Labs adjudication and findings were flawed, if not fraudulent. Comparative equipment trials were agreed and the event served to prove that the LL simulations were indeed erroneous in important respects.
Despite the exemplary performance of DMLS in the comparative trials, the US continued to denigrate the DMLS. AWOP members meeting April 1978 decided to ‘play safe’ and chose the more easily understood and conventional TRSB.
Other applications for commutated antennas were sought but none were found that looked like being profitable.
For the Cowes site and the Plessey Company this was not the end of the story. Determined to stay in the MLS market, Plessey Engineers set about using their knowledge and experience gained on DMLS, to design a commercial product, and so P-Scan was born.
P-Scan systems were installed at various airports all over the UK and spent an extended period at London Heathrow, being used to support ongoing operational data gathering by British Airways with the Boeing 757 Aircraft tail coded BIKK.
Meanwhile Plessey Engineers, (soon to become Siemens/Plessey Engineers) were working on the production variant, which included GaAs (Gallium Arsenide) active phased array technology in both the azimuth and elevation equipments. It was the most advanced MLS ever developed. The system was designated P-SCAN 2000, and the first launch sale of two systems was made in the 1980’s to UK National Air Traffic Services (NATS) for operational service at Heathrow. These systems were built at Cowes. Sadly the market was already rapidly declining and the USA was busy with a new ‘toy’ called GPS (Global Positioning Systems). They were convinced that this system could be used, free of charge to guide aircraft to land without the need for ground transmitters. Many around the world believed their marketing hype and the rug was pulled out from under MLS and to this day there are no CAT111 certified DGPS (Differential Global Positioning System) landing systems.
Forces were at work in the UK under the name of Airsys ATM, soon to become Thales ATM, also at Stuttgart, Germany (Sel Alcatel), to make MLS an operational reality. In collaboration with British Airways and NATS a further contract was placed for Cat 111 operational systems (quantity 4) for each of the approaches at Heathrow.
On the 25th March 2009, the Heathrow MLS installations received final certification from CAA Safety Regulation Group (SRG) to begin CAT 111b Operational use.
Today, British Airways are routinely flying MLS operations into Heathrow with their Air Bus fleet of 318 and 319 Aircraft. By comparison there is no known instance of GPS (even with its many and various augmentation systems) providing ‘sole-means’ precision landing guidance down to CAT 111 minima.
The DVOR (Doppler VHF Omni Directional range Radio Beacon) provides aircraft suitably equipped with a VOR receiver, with bearing information within a radius of 100nm from its known installed base. Distance information is also available, if a DME (Distance Measuring Equipment) is co-located with the DVOR. The PLAN 50 DVOR replaced the VOR systems widely in use throughout the world.
The whole system is capable of unattended remote switching and monitoring. A unique feature of the Plessey DVOR is the modular construction.
Plessey Radar (Cowes) first became involved with DVOR when the Civil Aviation Authority, now NATS, awarded a contract for the installation and commissioning of a DVOR system at Biggin Hill Aerodrome. The USA Federal Aviation Authority supplied the system under an exchange agreement. This installation was to be the first DVOR system operating in the UK. CAA conducted trials on the system over many months and on satisfaction sought tenders for the supply of DVOR systems that would become a UK-wide network replacing existing VOR systems. Gains for DVOR over VOR were to be improved reliability and improved signal integrity in the presence of multi-path*. The Biggin Hill DVOR was sited a few hundred metres away from an existing VOR system, enabling relative performance to be measured.
DVOR differs from VOR by virtue of its signal format. DVOR provides 360 degree angular coding in the azimuth plane in the form of a Doppler shift corresponding to the direction from which the installation is viewed, this by radiating a VHF signal from a source made to rotate rapidly on a horizontal circular path with a radius of around 30m. The received signal thus has a cyclic variation of Doppler shift with a phase specific to the bearing from which it is received. When received in an aircraft the actual bearing is determined by comparing the phase of the Doppler envelope with that of an amplitude-modulated reference signal transmitted from a stationary source at the centre of rotation.
The ‘rotating source’ antenna consists of a ring of closely spaced elements fed from a switch (commutator) that energises each in turn.
Experience gained was the platform for the design and development of the Plessey PLAN 50 DVOR that employed a circular array of 50 radiating elements.
More advanced technology and manufacturing techniques were features of the PLAN 50, in particular the electro-mechanical (spinning) commutator was replaced by a static electronic switch using PIN diodes. The (Alford loop) antenna elements were made by metal being sprayed onto a dielectric substrate instead of by the more labour intensive folded metal plate assembly.
*Multi-path is a phenomenon wherein a signal travels from the transmitter to the intended receiving location via an indirect path having been reflected from an object to one side of the direct line-of-sight. This can confuse the receiver in 3 ways.
- 1. The indirect path delays the signal and the information it carries.
- 2. Combination with the direct signal at the receiver causes fading.
- 3. The receiver gets a signal coded with the bearing of the reflecting object.
A VOR airborne receiver’s signal processor may be able to overcome these effects if the multi-path is of short duration. However, the task of separating the wanted (direct) signal from multi-path is much simplified if the angle coding is in the form of frequency rather than amplitude and this is largely why the integrity of Doppler VOR is superior.
Pictured is one switching disk of the PLAN 50 DVOR array commutator, shown in its partly built prototype form. Each ‘helix’ is a quarter wavelength line separating the two switching PIN diodes. RF, fed to the centre, is switched to the outer edge by a command signal arriving on the pink wire.
(This description of the CNTF belongs as part of Chapter 15.3. but has found book space here).
The CNTF was commissioned at the Cowes site in 1994. It replaced a far-field facility situated at Shorwell in the south of the Island, which was set up in the mid 1960’s. This range, with its main Transmitter at St. Catherine’s Down was overlong at 4.5miles, having a subtended angle of only 2degrees to the floor of the valley. The multipath problem could be accepted at that time, but as technology improved and low sidelobe antennas were developed, the far-field range was no longer suitable.
In the early 1990’s, it was decided to replace the Shorwell site by a near-field facility. A conventional near-field scanner built inside an anechoic chamber was cost prohibitive when allowing for the larger antennas, so it was decided to build an outside facility. The cylindrical format was chosen over spherical or planar as most suitable for the radar systems being built at Cowes at that time. Deutsche Aerospace was awarded the contract to design and build the facility. It is capable of fully characterising all antennas operating in the 1GHz - 6GHz frequency range, maximum 9.5 m diameter and 3000 Kg in weight.
Principle of operation
A cylinder of near-field data is measured around the antenna under test and processed to provide the far field radiation characteristics. The antenna is rotated up to 6 rpm and the field sampled using a probe mounted on the CNTF's 36m high tower. A discrete ring of data is acquired, with the probe being moved incrementally upwards with the process repeated until the complete cylinder of near-field data is generated for processing.
The diagnostic capabilities of the CNTF allow the field at the aperture to be predicted. This permits the rapid identification of any defects in the antenna.
The CNTF incorporates many advanced technologies. These include the design of the low radar cross-section profile of the high stability tower and the laser controlled positioning mechanism of the RF probe, providing extremely high spatial accuracy. The vertical laser beams are mounted on a real-time compensating one tonne granite block at the foot of the tower which has feed-back from the rotational axis of the turntable and can position the RF probe within 0.5mm at half tower height.
The CNTF is a very complicated piece of equipment and, being a prototype, proved very temperamental in its early years. For testing simple antennas, it was like using a sledgehammer to crack a nut, but for the more advanced arrays, it proved its worth, especially with its diagnostic capabilities. In the early days, the test specifications often had to be amended, as they were written around the inaccuracies of the measurements at the Shorwell’s far-field range. The CNTF was too accurate!
- Sidelobe: +/- 0.75dB
- 3dB Beamwidth +/-0.05 deg.
- Gain: +/- 0.25dB
- Beam Pointing +/- 0.1 deg.